Method for high efficiency protein delivery into plastids

ABSTRACT

The present invention provides a recombinant DNA molecule encoding a fusion protein, comprising a first DNA sequence encoding a high-efficiency transit peptide operably linked to a second DNA sequence encoding a passenger protein, wherein the high-efficiency transit peptide is selected from the group consisting of transit peptides of the precursors of translocon at the inner envelope membrane of chloroplasts 40 kD (prTic40), chaperonin 10-2 (prCpn10-2), Fibrillin 1B (prFibrillin), ATP sulfurylase 1 (prAPS1), ATP sulfurylase 3 (prAPS3), 5′-adenylylsulfate reductase 3 (prAPR3), stromal ascorbate peroxidase (prsAPX), prTic40-E2A (a prTic40 variant), prCpn10-1-ΔC7C37S (a chaperonin 10-1 variant), a functional fragment of any of the transit peptides and an equivalent thereof. And the present invention also provides a method of high efficiency delivery of a protein into plastids using the high-efficiency transit peptides.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Divisional of co-pending U.S. application Ser. No. 15/210,445, filed on Jul. 14, 2016, which claims the priority benefit of Provisional Application No. 62/192,393, filed on 14 Jul. 2015; the entire contents of all of the above-identified applications are incorporated herewith by reference.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to plant biotechnology. More particularly, the present invention relates a method for high efficiency protein delivery into plastids, in particular, leucoplasts.

2. The Prior Arts

Plastids are essential organelles in plants responsible for functions ranging from photosynthesis, biosynthesis of all fatty acids, starch, carotenoids, and most amino acids, to assimilation of nitrogen and sulfur. Plastids differentiate into different functional types in different tissues, for example chloroplasts in green tissues for photosynthesis, chromoplasts in petals and fruits for carotenoid pigments accumulation, and leucoplasts in non-green tissues for synthesis and storage of nutrients, including starch, proteins and oils. To perform these specific functions, different types of plastids require, and therefore import, different proteins.

Although plastids have their own genome, most plastid proteins are encoded by the nuclear genome, synthesized in the cytosol as a larger precursor with an N-terminal extension called the transit peptide. Transit peptides are necessary and sufficient for targeting passenger proteins into plastids, i.e., a transit peptide can be taken from the original precursor and fused to a passenger protein and results in delivery of the passenger protein into plastids. Transit peptides that can deliver passenger proteins into chloroplasts with high efficiency, for example the transit peptide of RuBP carboxylase small subunit precursor (prRBCS), have been identified and used to deliver passenger proteins into plastids.

Most grain- and root-type food crops, for example rice, corn and cassava, use leucoplasts to synthesize and store the starch that is used to feed the majority of the world population. However, despite the economic importance of leucoplasts, almost all of our knowledge about plastid protein import is derived from studies with chloroplasts and little is known about how proteins are imported into leucoplasts. Unfortunately, leucoplasts clearly have a different substrate preference, and transit peptides like that of prRBCS import proteins poorly into leucoplasts. For example, it has been shown that the transit peptide of prRBCS could not direct the import of the passenger protein green fluorescent protein (GFP) into leucoplasts in endosperms of transgenic wheat (Primavesi et al., 2008). Using leucoplasts isolated from castor seeds and chloroplasts isolated from pea, it has been shown that prRBCS imported much better into chloroplasts than into leucoplasts (Wan et al., 1996). Using leucoplasts and chloroplasts isolated from pea roots and leaves, respectively, it has been shown that prRBCS could not be imported into leucoplasts at all (Yan et al., 2006). Nonetheless, in these studies, no proper quantitative comparisons were performed so the import efficiency of transit peptides could not be compared directly among one another. Therefore the exact import efficiency of prRBCS transit peptide into leucoplasts is not known and no transit peptides with high leucoplast import efficiency have been discovered.

The transit peptide for prRBCS is the most widely used transit peptide for delivering engineered proteins into plastids in biotechnology applications. Examples include the famous Golden Rice, Roundup Ready® corn and Dicamba resistant soybean. As discussed above, many reports have suggested that this transit peptide deliver proteins poorly into leucoplasts and therefore is not the best transit peptide for applications that need to express proteins in leucoplasts. However, no report has performed quantitative comparisons between transit peptides. If quantitative comparison can be performed and transit peptides with higher leucoplast import efficiency than prRBCS transit peptide can be identified, these new transit peptides would be valuable tools for delivering engineered proteins into leucoplasts.

SUMMARY OF THE INVENTION

The present invention relates to a recombinant DNA molecule encoding a fusion protein comprising a first DNA sequence encoding a high-efficiency transit peptide operably linked to a second DNA sequence encoding a passenger protein, a DNA construct comprising said recombinant DNA molecule operably linked to a promoter, a plant material comprising said DNA construct or said recombinant DNA molecule, and a method for high efficiency protein delivery into plastids, in particular, leucoplasts, using said recombinant DNA molecule or said construct.

Leucoplasts are plastids essential for the synthesis and storage of starch, lipids and proteins. Most grain and root-type crops use leucoplasts to synthesize and store nutrients. Modifications of the biosynthesis processes within leucoplasts to better suit human needs will require high efficiency delivery of engineered proteins into leucoplasts. Many reports have shown that proteins that could import efficiently into chloroplasts import poorly into leucoplasts because the two plastids have different protein preference. However, most plastid-targeting transit peptides currently available are derived from precursor proteins with high chloroplast import efficiency. No transit peptide with high leucoplast import efficiency is available. To identify transit peptides with high leucoplast import efficiency, the present invention first optimized the in vitro leucoplast import system to allow fast, quantitative and accurate comparisons of import efficiencies of a large number of plastid precursor proteins. Using this leucoplast import system, the present invention identified a group of nine precursor proteins that have very high import efficiency into leucoplasts and chloroplasts. These precursors imported into chloroplasts equally well or better than prRBCS, but imported into leucoplasts two to eight times better than prRBCS, whose transit peptide is currently the most widely used transit peptide for directing engineered proteins into all plastids, including leucoplasts.

An objective of the present invention is to provide a recombinant DNA molecule encoding a fusion protein, wherein said recombinant DNA molecule comprising a first DNA sequence encoding a high-efficiency transit peptide operably linked to a second DNA sequence encoding a passenger protein, wherein the high-efficiency transit peptide is selected from the group consisting of the transit peptides of the precursors of translocon at the inner envelope membrane of chloroplasts 40 kD (prTic40), chaperonin 10-2 (prCpn10-2), Fibrillin 1B (prFibrillin), ATP sulfurylase 1 (prAPS1), ATP sulfurylase 3 (prAPS3), 5′-adenylylsulfate reductase 3 (prAPR3), stromal ascorbate peroxidase (prsAPX), prTic40-E2A (a prTic40 variant), prCpn10-1-ΔC7C37S (a chaperonin 10-1 variant), a functional fragment of any of said transit peptides and an equivalent thereof.

Another objective of the present invention is to provide a DNA construct comprising the recombinant DNA molecule encoding a fusion protein as described herein, operably linked to a promoter.

Another objective of the present invention is to provide a plant material transformed with, and comprising the DNA construct.

Another objective of the present invention is to provide a method of high efficiency delivery of a passenger protein into plastids of the transformed plant material, comprising: providing a recombinant DNA molecule encoding a fusion protein; linking the recombinant DNA molecule operably with a promoter to form a DNA construct; and transforming the DNA construct into a plant material to express the fusion protein, wherein the recombinant DNA molecule comprises a first DNA sequence encoding a high-efficiency transit peptide operably linked to a second DNA sequence encoding the passenger protein, wherein the high-efficiency transit peptide is selected from the group consisting of the transit peptides of the precursors of translocon at the inner envelope membrane of chloroplasts 40 kD (prTic40), chaperonin 10-2 (prCpn10-2), Fibrillin 1B (prFibrillin), ATP sulfurylase 1 (prAPS1), ATP sulfurylase 3 (prAPS3), 5′-adenylylsulfate reductase 3 (prAPR3), stromal ascorbate peroxidase (prsAPX), prTic40-E2A (a prTic40 variant), prCpn10-1-ΔC7C37S (a chaperonin 10-1 variant), a functional fragment of any of said transit peptides and an equivalent thereof.

In one embodiment of the present invention, the first DNA sequence encoding a high-efficiency transit peptide is selected from the group consisting of SEQ ID NO:10 to SEQ ID NO:18, a functional fragment of any of said SEQ ID NOs and an equivalent thereof.

In one embodiment of the present invention, the high-efficiency transit peptides as described herein are capable of delivering an amount of passenger proteins that is equal or higher than the amount of passenger proteins a prRBCS transit peptide is capable of delivering into chloroplasts, and are capable of delivering an amount of passenger proteins that is higher than the amount of passenger proteins a prRBCS transit peptide is capable of delivering into leucoplasts.

In one embodiment of the present invention, the passenger protein is a biologically-active protein. In another embodiment, the passenger protein is a protein to be delivered to plastids.

In one embodiment of the present invention, the promoter is functional in a plant cell.

In one embodiment of the present invention, the plant material is selected from the group consisting of a plant cell, a plant tissue, a plant tissue culture, a callus culture and a transgenic plant.

In one embodiment of the present invention, the plant material is obtained from a monocotyledon or a dicotyledon plant.

Accordingly, the nine transit peptides of the present invention, including transit peptides of the precursors of the translocon at the inner envelope membrane of chloroplasts 40 kD (prTic40) (SEQ ID NO:10), chaperonin 10-2 (prCpn10-2) (SEQ ID NO:11), Fibrillin 1B (prFibrillin) (SEQ ID NO:12), ATP sulfurylase 1 (prAPS1) (SEQ ID NO:13), ATP sulfurylase 3 (prAPS3) (SEQ ID NO:14), 5′-adenylylsulfate reductase 3 (prAPR3) (SEQ ID NO:15), stromal ascorbate peroxidase (prsAPX) (SEQ ID NO:16), prTic40-E2A (a prTic40 variant) (SEQ ID NO:17) and prCpn10-1-ΔC7C37S (a chaperonin 10-1 variant) (SEQ ID NO:18), provide a valuable tool to enable high efficiency delivery of engineered proteins into plastids for all kinds of plants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show that prRBCS imported much less efficiently into leucoplasts than into chloroplasts no matter whether the comparison was done based on equal proteins or on equal plastids, and prTic40 had much higher leucoplast import efficiencies than prRBCS. In FIG. 1A, leucoplasts and chloroplasts were isolated from roots of 4-day-old pea seedlings grown in the dark and leaves of 7-day-old pea seedlings grown under 12-h photoperiod, respectively. The isolated leucoplasts (500 or 113.67 μg protein) and chloroplasts (500 μg protein) were incubated under import conditions with 18 μL in vitro-translated [³⁵S]Met-prTic40 or [³⁵S]Met-prRBCS and 3 mM ATP in import buffer in a final volume of 200 μL for 25 min. The two different concentrations of leucoplasts, 500 and 113.67 μg of protein, are for comparing with chloroplasts based on equal amounts of proteins and equal numbers of plastids, respectively. 500 μg of leucoplasts have the same amount of proteins as 500 μg of chloroplasts, while 113.67 μg of leucoplasts have the same number of plastids as 500 μg of chloroplasts. After import, intact leucoplasts and chloroplasts were re-isolated through a 10% and 40% Percoll cushion, respectively. The import samples were analyzed by SDS-PAGE, stained with Coomassie blue, and dried for fluorography. 4% of the plastids in each import reaction (20 μg or 4.55 μg of proteins) were loaded. Tr, 1% of the in vitro-translated precursor proteins used in each import reaction. In FIG. 1B, the imported mature proteins in experiments as those shown in FIG. 1A were quantified and the import efficiencies were calculated. Data shown are mean±SD of three independent experiments. Cpt, chloroplasts; Leu, leucoplasts. pr, precursor protein; m, mature protein derived after import.

FIGS. 2A and 2B show that different precursor proteins had different plastid preference. The precursor prFd-protein A serves as an example of highly preferring chloroplasts. The precursor prPDH E1α serves as an example of mildly preferring chloroplasts. The precursor prCpn10-2 serves as an example of showing no preference and being able to import well into both chloroplasts and leucoplasts. In FIG. 2A, isolated leucoplasts (113.67 μg protein) and chloroplasts (500 μg protein) were incubated with in vitro-translated prFd-protein A, prPDH E1α, or prCpn10-2 under import conditions for 25 min. After import, intact leucoplasts and chloroplasts were re-isolated through a 10% and 40% Percoll cushion, respectively. The import samples were analyzed by SDS-PAGE, stained with Coomassie blue, and dried for fluorography. 4% of the plastids in each import reaction were loaded. Tr, 1% (for prFd-protein A and prPDH E1α) or 1.2% (for prCpn10-2) of the in vitro-translated precursor proteins used in the import reactions. In FIG. 2B, imported mature proteins in experiments as those shown in FIG. 2A were quantified and the import efficiencies were calculated. Data shown are mean±SD of three independent experiments. Cpt, chloroplasts; Leu, leucoplasts. pr, precursor protein; m, mature protein derived after import.

FIGS. 3A to 3C show that nine precursor proteins had high import efficiencies into both leucoplasts and chloroplasts, and these nine precursor proteins all had much higher leucoplast import efficiency than prRBCS. In FIG. 3A, isolated leucoplasts (113.67 μg proteins) and chloroplasts (500 μg proteins) were incubated with in vitro-translated [³⁵S]Met-precursor proteins under import conditions for 25 min. After import, intact leucoplasts and chloroplasts were re-isolated through a 10% and 40% Percoll cushion, respectively. The samples were analyzed by SDS-PAGE, stained with Coomassie blue, and dried for fluorography. 4% of the plastids in each import reaction were loaded. Tr, 1.2% of the in vitro-translated precursor proteins used in each import reaction. pr, precursor protein; m, mature protein derived after import. In FIGS. 3B and 3C, imported mature proteins in chloroplasts (3B) and leucoplasts (3C) in experiments as those shown in FIG. 3A were quantified and the import efficiency of each precursor was calculated. The import efficiency of prRBCS was set as 1. Data shown are mean±SD of at least three independent experiments. Cpt, chloroplasts; Leu, leucoplasts.

FIGS. 4A and 4B show that the nine high-efficiency transit peptides of the present invention indeed delivered a higher amount of passenger proteins into leucoplasts than the prRBCS transit peptide. In FIG. 4A, protoplasts isolated from tobacco BY2 cells were co-transformed with GFP or transit-peptide-GFP fusion plasmids and the plasmid pBI221, which directed the expression of β-glucuronidase (GUS) in the cytosol and served as an internal control for transformation and protein expression efficiency. After transformation, the protoplasts were incubated in the dark at 25° C. for 16 h, analyzed by SDS-PAGE and immunodetected with antibodies indicated on the left. Thirty micrograms of proteins were loaded in each lane. Plasmid DNA used for each lane is labeled at the top. Lane 1, control protoplasts with no plasmid DNA added during the transformation. Lane 2, protoplasts transformed with pBI221 and the GFP vector without fusing to any transit peptide. Lanes 3 to 12, protoplasts transformed with pBI221 and a plasmid encoding GFP fused to prRBCS transit peptide (lane 3), or GFP fused to one of the nine transit peptides of the present invention (lanes 4 to 12). The transit peptide part is indicated with subscript “tp”. Filled circles mark the precursor form of each fusion protein and brackets mark GFP imported into BY2 cell leucoplasts. In FIG. 4B, the amount of GUS and imported GFP in experiments as those shown in FIG. 4A were quantified. The efficiency of each transit peptide was calculated by the amount of GFP normalized to the amount of GUS in the same sample. The GFP/GUS ratio of RBCS_(tp)-GFP was set as 1. Data shown are mean±SD of two independent experiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

As a first step in identifying precursors with high leucoplast import efficiency, the present invention optimized the in vitro leucoplast protein import system for higher efficiency import and fast, accurate and quantitative comparisons, between leucoplasts and chloroplasts, and among transit peptides. A group of nine precursors that imported into chloroplasts equally well as prRBCS, but imported into leucoplasts much more efficiently than prRBCS, were identified. The present invention further showed that a higher amount of passenger proteins were imported into leucoplasts in vivo when the passenger protein was fused to these high efficiency transit peptides of the present invention, than when fused to the prRBCS transit peptide.

Definition

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.

As used herein, promoters suitable for the practice of this invention include all promoters that have been shown to drive RNA expression in plants, including those that are constitutive, inducible or tissue-specific. Examples include, but are not limited to, maize RS81 promoter, rice ubiquitin promoters, rice glutelin (Gt1) promoter, maize RS324 promoter, maize PR-1 promoter, maize A3 promoter, maize L3 oleosin promoter, rice actin promoters, prRBCS promoter, phytoene desaturase promoter, sporamin promoter, gamma-coixin promoter, maize chloroplast aldolase promoter, nopaline synthase (NOS) promoter, octopine synthase (OCS) promoter, cauliflower mosaic virus (CaMV) 19S and 35S promoters, figwort mosaic virus 35S promoter, Arabidopsis sucrose synthase promoter, R gene complex promoter, chlorophyll a/b binding protein gene promoter, CaMV35S promoter with enhancer sequences e35S, FMV35S promoter, FLt36 promoter of peanut chlorotic streak virus, At.Act 7 promoter, At.ANT1 promoter, FMV.35S-EF1a promoter, eIF4A10 promoter, AGRtu.nos promoter, and rice cytosolic triose phosphate isomerase promoter.

As used herein, passenger proteins suitable for the practice of this invention are any protein the one of ordinary skill in the art wishes to express in plastids. Examples include, but are not limited to, GFP, β-glucuronidase (GUS), dicamba monooxygenase (DMO), enolpyruvyl shikimate-3-phosphate (EPSP) synthase, glyphosate oxidase (GOX), phytoene synthase (psy), AtOR (R90H) (the R90H mutant of Arabidopsis ORANGE protein), β-carotene ketolase and phytoene desaturase.

As used herein, “monocotyledon plant” refers to any of a class of angiosperm plants having a single cotyledon in the seed, and includes (but is not limited to) rice, rye, wheat, barley, sorghum, maize, oat, orchids, lily, banana, taro and sugar cane.

As used herein, “dicotyledon plant” refers to an angiosperm that is not a monocotyledon, having two cotyledons in the seed, and includes (but is not limited to) Arabidopsis, tobacco, potato, sweet potato, canola, soybean, bean, cotton, sunflower, white cauliflower, chrysanthemum, cassava and roses.

As used herein, a “high-efficiency” transit peptide refers to a transit peptide which is capable of delivering an amount of passenger proteins that is equal or higher than the amount of passenger proteins a prRBCS transit peptide is capable of delivering into chloroplasts, and is capable of delivering an amount of passenger proteins that is higher than the amount of passenger proteins a prRBCS transit peptide is capable of delivering into leucoplasts. The method used for comparing the amounts of passenger proteins being delivered into different groups of chloroplasts and leucoplasts can be the optimized method as described herein, or any such method that is capable of differentiating between a higher and a lower amount of a particular protein that has accumulated in chloroplasts and leucoplasts of at least two experimental groups of cells.

As used herein, the terms “construct” and “vector” are used interchangeably.

Transit peptides are known to function across species. For example, the prRBCS transit peptide from pea was used in the Golden rice creation. The pea prRBCS transit peptide is also used in creating the Dicamba resistant soybean and the initial studies for the creation were performed in Arabidopsis and tobacco. The prRBCS transit peptide from Arabidopsis was used in the Roundup Ready® corn (USDA, 1996). A transit peptide from the waxy protein of corn was shown to function well in potato. For the current invention, the high-efficiency transit peptides were identified from pea and Arabidopsis and the initial tests were performed in pea, tobacco and rice and will also be tested in Arabidopsis. It is expected that these nine transit peptides of the present invention will provide high efficiency delivery of proteins into leucoplasts of all monocotyledon and dicotyledon plants, for example leucoplasts in the tubers, endosperms and roots of rice, barley, wheat, corn, cassava, potato and soybean and into leucoplasts of white colored petals of all flowers. Furthermore, because precursors containing these transit peptides also import very efficiently into chloroplasts, these transit peptides will also be expected to deliver proteins into chloroplasts with high efficiency.

Example 1 Plastid Isolation, Protein Concentration Assays, and Plastid Number Counting

Pea seedlings (Pisum sativum cv. Green Arrow) were grown at 20° C. on vermiculite. Leucoplasts were prepared from roots of 4- to 5-day-old dark grown seedlings as described (Chu and Li, 2015). Chloroplasts were isolated from leaves of 7-day-old seedlings grown under a 12-h photoperiod with a light intensity of approximately 150 μmol m⁻² s⁻¹ as described (Perry et al., 1991), except 2 mM ascorbic acid, 0.1 mM dithiothreitol, and 1.2 mM glutathione were added in the grinding buffer used for homogenization. Isolated chloroplasts were adjusted to 1 mg chlorophyll mL⁻¹ in import buffer.

Example 2 Optimization of the Protocol for Isolation of Import-Competent Leucoplasts

To set up a quantitative leucoplast import system, the present invention first optimized the conditions for leucoplast isolation. The present invention increased the concentration of EDTA and BSA in the homogenization buffer and also added reducing agents into the buffer. After these modifications, the import efficiencies of precursor proteins into isolated leucoplasts were increased (Chu and Li, 2015).

Example 3 Plasmid Construction and In Vitro Translation of Precursors for In Vitro Import into Isolated Plastids

Plasmids encoding prRBCS, prTic40, prFd-protein A, prPDH E1α, and prCpn10-2 have been described (Teng et al., 2012). The cDNA clones of pda02149 for precursor of ATP sulfurylase 1 (prAPS1, AT3G22890) and pda04912 for precursor of 5′-adenylylsulfate reductase 3 (APR3, AT4G21990) were obtained from RIKEN BioResource Center. The leaf cDNA pools of Arabidopsis thaliana (Columbia ecotype) were used as templates to amplify the coding regions of Fibrillin 1B precursor (prFibrillin, AT4G22240), ATP sulfurylase 3 precursor (prAPS3, AT4G14680), and stromal ascorbate peroxidase precursor (prsAPX, AT4G08390) with specific forward and reverse primer pairs as described in Table 1. The PCR products of prFibrillin were digested with HindIII and PstI and cloned into the HindIII/PstI of pSP72. The PCR products of prAPS3 and prsAPX were digested with XhoI and SalI and cloned into the XhoI/SalI site of pSP72, respectively. The sequences of prFibrillin, prAPS3, and prsAPX were confirmed by sequencing and the plasmid was named pSP72-Fibrillin, pSP72-APS3, and pSP72-sAPX, respectively. Since the prFibrillin has only two Met in the transit peptide region, the sequence encoding 2 extra Met were inserted into the 3′end of cDNA before the stop codon using the QuikChange II Site-Directed Mutagenesis Kit (AGILENT TECHNOLOGIES) with primers fibrillin-2M-F and fibrillin-2M-R (Table 1). The sequence was confirmed by sequencing and the plasmid was named pSP72-Fibrillin-2M and this plasmid was used in the subsequent analyses. The prTic40 cording region in the pBS plasmid was digested with XhoI and PstI and cloned into the XhoI/PstI site of pSP72 to generate the pSP72-Tic40 plasmid. This pSP72-Tic40 construct was used to generate the prTic40 variant using the QuikChange II Site-Directed Mutagenesis Kit (AGILENT TECHNOLOGIES) with primers pSP72-Tic40-E2A-F and pSP72-Tic40-E2A-R (Table 1). The sequence was confirmed by sequencing and the plasmid was named pSP72-Tic40-E2A. For the prCpn10-1 variant, prCpn10-1-ΔC7C37S, the prCpn10-1 plasmid (Teng et al., 2012) was used for site-directed mutagenesis via the QuikChange II Site-Directed Mutagenesis Kit (AGILENT TECHNOLOGIES) with primers Cpn10-1-ΔC7-F and Cpn10-1-ΔC7-R to generate the plasmid pSP72-Cpn10-1-ΔC7 first. Then pSP72-Cpn10-1-ΔC7 was used for site-directed mutagenesis with primers Cpn10-1-C37S-F and Cpn10-1-C37S-R. The sequence was confirmed by sequencing and the plasmid was named pSP72-Cpn10-1-ΔC7C37S. The in vitro expression of prAPS1 and prAPR3 was under the control of the T7 promoter. The in vitro expression of prFibrillin, prAPS3, prsAPX, prTic40-E2A, and prCpn10-1-ΔC7C37S was under the control of the SP6 promoter.

[³⁵S]Met-labeled prPDH E1α was generated by in vitro transcription for synthesizing RNA followed by in vitro translation using the Rabbit Reticulocyte Lysate system (PROMEGA). All other precursors were synthesized using the TNT Coupled Wheat Germ Extract system or TNT Coupled Reticulocyte Lysate system (PROMEGA).

TABLE 1 Primers used for cloning in the invention Primer Nucleotide sequence Purpose fibrillin- 5′-cgaagcttatggcgacggtacaattgtc-3′ To clone the coding region of F1-HindIII SEQ ID NO: 21 prFibrillin into pSP72 fibrillin- 5′-cgctgcagtcaaggattcaagagagg-3′ To clone the coding region of R1-PstI SEQ ID NO: 22 prFibrillin into pSP72 APS3-F1- 5′-cactcgagatggcttccatgtccaccgtcttcc-3′ To clone the coding region of XhoI SEQ ID NO: 23 prAPS3 into pSP72 APS3-R1- 5′-cagtcgacttaaaccggaatcttttccggaagtt-3′ To clone the coding region of SalI SEQ ID NO: 24 PrAPS3 into pSP72 sAPX-F2- 5′-agctcgagatggcagagcgtgtgtctc-3′ To clone the coding region of XhoI SEQ ID NO: 25 prsAPX into pSP72 sAPX-R2- 5′-gcgtcgacttagataacgataccctccg-3′ To clone the coding region of SalI SEQ ID NO: 26 prsAPX into pSP72 fibrillin- 5′-tctcttgaatcctatgatgtgactgcaggtcg-3′ To add two extra methionine 2M-F SEQ ID NO: 27 residues in the C terminus of fibrillin fibrillin- 5′-cgacctgcagtcacatcataggattcaagaga-3′ To add two extra methionine 2M-R SEQ ID NO: 28 residues in the C terminus of fibrillin pSP72- 5′-cgataagcttgatatggcgaatcttaacttagccc- To mutate the glutamic acid Tic40- 3′ residue at position 2 of E2A-F SEQ ID NO: 29 prTic40 into alanine residue pSP72- 5′-gggctaagttaagattcgccatatcaagcttatcg- To mutate the glutamic acid Tic40- 3′ residue at position 2 of E2A-R SEQ ID NO: 30 prTic40 into alanine residue Cpn10-1- 5′-gcttccactttcgtctctctaccaaatcct-3′ To delete the cysteine residue ΔC7-F SEQ ID NO: 31 at position 2 of prCpn10-1 Cpn10-1- 5′-aggatttggtagagagacgaaagtggaagc-3′ To delete the cysteine residue ΔC7-R SEQ ID NO: 32 at position 2 of prCpn10-1 Cpn10-1- 5′-cggaagtcgaagaggttcccttagaatcaaagcga- To mutate the cysteine residue C37S-F 3′ at position of 37 of prCpn10-1 SEQ ID NO: 33 into serine Cpn10-1- 5′-tcgctttgattctaagggaacctcttcgacttccg- To mutate the cysteine residue C375-R 3′ at position of 37 of prCpn10-1 SEQ ID NO: 34 into serine

Example 4 Protein Import into Isolated Plastids and Post-Import Analyses

After setting up the leucoplast isolation conditions, the present invention next compared the import behavior of various precursor proteins into leucoplasts and chloroplasts. An equal amount of proteins (Yan et al., 2006) or an equal number of plastids (Wan et al., 1996) was usually used as the basis when comparing among import efficiencies of different plastids. To determine the number of plastids, the isolated leucoplasts and chloroplasts were counted using the Multisizer 3 Coulter Counter (BECKMAN COULTER). The present invention used chloroplasts isolated from 7-day-old seedlings for robust import and for decreasing the age difference between the leucoplast and chloroplast samples. The average size of the isolated leucoplasts and chloroplasts was estimated to be 1.81±0.21 μm and 3.24±0.23 μm, respectively. The same plastid preparations were then used for protein concentration determination and the average protein content was calculated to be 1.85±0.63 and 8.13±1.42 μg/plastid for leucoplasts and chloroplasts, respectively. The present invention then used protein content as an estimate of plastid numbers in the experiments thereafter. For example, 500 μg of plastid proteins would represent approximately 2.71×10⁸ leucoplasts and 6.15×10⁷ chloroplasts. For import comparison between chloroplasts and leucoplasts on an equal protein basis, 18 μL [³⁵S]Met-labeled precursors were incubated with isolated plastids equivalent to 500 μg plastid proteins in the presence of 3 mM ATP in import buffer in a final volume of 200 μL. For import comparison on an equal plastid number basis, 113.67 μg leucoplast proteins (2.71×10⁸ leucoplasts) and 500 μg chloroplast proteins (2.71×10⁸ chloroplasts) were used instead. Import reactions were carried out at room temperature for 25 mM and stopped by transferring to a new tube containing 1 mL ice-cold import buffer. The plastids were pelleted by centrifugation at 3,000 g at 4° C. for 3 mM and re-suspended in 200 μL import buffer. The leucoplast suspensions were underlaid with 1 mL 10% Percoll™ (v/v) in import buffer and the chloroplast suspensions were laid on top of a 40% Percoll™ (v/v) cushion to re-isolate the intact plastids with a swinging-bucket rotor by 2,900 g for 6 mM at 4° C. The plastids were washed once with import buffer and re-suspended in a small volume of import buffer. Protein concentrations of the plastid samples were measured with the BCA protein assay kit (THERMO). Samples were analyzed by SDS-PAGE. Quantification of gel bands was performed using the Fuji FLA5000 PhosphorImager (FUJIFILM, Tokyo).

Example 5 prRBCS Imported Poorly into Leucoplasts and prTic40 Imported Efficiently into Both Chloroplasts and Leucoplasts

To provide quantitative comparison of the import efficiency of prRBCS into chloroplasts and leucoplasts, import was compared based on equal proteins or on equal plastid numbers. The results showed that the import efficiency of prRBCS into leucoplasts was much lower than its import efficiency into chloroplasts no matter whether the comparison was done on an equal amount of proteins or on an equal number of plastids basis (FIGS. 1A and 1B). Furthermore, in initial screenings from our original collection of precursor proteins, prTic40 showed the best import activity into isolated leucoplasts and it was therefore used to compare with prRBCS. prTic40 imported into chloroplasts with similar efficiency to prRBCS, and imported into leucoplast much more efficiently than prRBCS.

Example 6 Identification of Transit Peptides with High Leucoplast Import Efficiencies

The present invention further tested another three precursors, ferredoxin precursor (prFd), pyruvate dehydrogenase E1α subunit precursor (prPDH E1α), and chaperonin 10-2 (prCpn10-2) precursor, to confirm that different precursors have different plastid preferences. For prFd, the present invention used the construct prFd-protein A, which contains ferredoxin transit peptide fused to Staphylococcal protein A, as described (Smith et al., 2004). As shown in FIGS. 2A and 2B, prFd-protein A imported efficiently into chloroplasts but minimally imported into leucoplasts, similar to the results of prRBCS. In comparison, although prPDH E1α also imported more efficiently into chloroplasts, but its import into leucoplasts was about half that of chloroplasts. In comparison, prCpn10-2 imported equally well into both chloroplasts and leucoplasts.

Precursors like prTic40 and prCpn10-2 are of particular interests because they can import very efficiently into both chloroplasts and leucoplasts. The present invention further cloned and tested the import of more than 60 plastid precursor proteins from Arabidopsis in the leucoplast import system the present invention set up. The present invention found another five precursors that also exhibited very high import efficiency into both chloroplasts and leucoplasts (FIGS. 3A to 3C and Table 2). They include precursors to Fibrillin 1B (prFibrillin), ATP sulfurylase 1 (prAPS1) and 3 (prAPS3), 5′-adenylylsulfate reductase 3 (prAPR3) and stromal ascorbate peroxidase (prsAPX). Another two precursor variants, prTic40-E2A and prCpn10-1-ΔC7C37S, also showed high import efficiency into both chloroplasts and leucoplasts (FIGS. 3B and 3C and Table 2). For these nine precursors, their chloroplast import efficiency was comparable to that of prRBCS (FIG. 3B), but their leucoplast import efficiency was 2-8 times higher than that of prRBCS (FIG. 3C).

TABLE 2 Transit peptides with high import efficiencies in plastids. Amino acids of the transit peptide are shown in lowercase letters and the first three amino acids in the mature region are shown in capital letters. The prRBCS transit peptide used as a control is also shown here. Precursors (Accession Transit peptide number) Full name Nucleotide sequences sequence prTic40 Translocon  atggagaatc ttaacttagc ccttgtttct  menlnlalvs spkplllghs (AY157668) at the inner  tcccctaaac ccctgctttt aggacattcc ssknvfsgrk sftfgtfrvs envelope tcctcaaaaa acgttttctc aggaaggaag ansssshvtr aaskshqnlk membrane of tctttcactt ttgggacgtt tcgcgtttct svqgkvnahd FAS chloroplasts  gctaactctt catcctctca tgtcaccagg SEQ ID NO: 10 40 kD gctgcttcta aatctcacca aaatctaaaa tctgtgcagg ggaaggtgaa tgcgcatgat tttgctagc SEQ ID NO: 1 prCpn10-2 chaperonin  atggcttcga gtttcattac agtacctaaa  massfitvpk pflsfpiktn (AT3G60210) 10-2 cccttcttgt ccttccccat caaaaccaat  aptlpqqtll girrnsfrin gctcctactc tacctcagca gacccttctc AVS ggaattcgaa gaaattcctt tagaattaac SEQ ID NO: 11 gccgtttcc SEQ ID NO: 2 prFibrillin  Fibrillin  atggcgacgg tacaattgtc cacccaattt  matvqlstqf scqtrvsisp 1B 1B agctgccaaa ccagagtttc aatctcaccg  nsksiskppf lvpvtsiihr (AT4G22240) aactctaaat ctatctccaa gcctccgttt pmistggiav sprrvfkvra ctggtaccgg tgacctcaat tattcaccgc tdtgeigsal laaEEA ccgatgatct ccaccggagg aatcgctgtt  SEQ ID NO: 12 tccccccgta gagttttcaa agtccgagcc  acagatacgg gagagatagg atcagctcta ttggcggcgg aggaagca SEQ ID NO: 3 prAPS1 ATP  atggcttcaa tggctgccgt cttaagcaaa  masmaavlsk tpflsqpltk (AT3G22890) sulfurylase  actccattcc tctctcaacc actaaccaaa  sspnsdlpfa avsfpskslr 1 tcatctccaa actccgatct ccccttcgcc rrvgsiragl iapdggklve gcggtttcct tcccttccaa atccctacgc liveepkrre kKHE cgccgcgtag gatcaatccg agccggatta  SEQ ID NO: 13 atcgctcccg acggtggtaa gcttgtagag  ctcatcgtgg aagagccaaa gcggcgagag  aagaaacacg ag SEQ ID NO: 4 prAPS3 ATP  atggcttcca tgtccaccgt cttccccaaa  masmstvfpk ptsfisqplt (AT4G14680) sulfurylase  ccaacctctt tcatctctca acctctaaca  kshksdsvtt sisfpsnskt 3 aaatctcaca aatccgattc cgtaaccaca rslrtisvra gliepdggkl tccatttcat tcccttcgaa ttcgaaaact vdlvvpeprr rEKK cgtagcttaa gaaccatctc tgtacgagct  SEQ ID NO: 14 ggcttaatcg agccagatgg tgggaaactt gtggatcttg ttgtaccgga accgagacgg cgagagaaga aa SEQ ID NO: 5 prAPR3 5′- atggcactag caatcaacgt ttcttcatct  malainvsss sssaissssf (AT4G21990) adenylyl- tcttcttctg cgatctcaag ctctagcttc  pssdlkvtki gslrllnrtn sulfate ccttcttcag atctcaaagt aacaaaaatc vsaaslslsg krssvkalnv reductase 3 ggatcattga ggttattgaa tcgtaccaat qsitkesiva sevtekldvv gtctctgcgg cttctctgag tttgtccggg  EVE aagagatcct ccgtgaaagc tcttaatgtg  SEQ ID NO: 15 caatcaatta caaaggaatc cattgttgct tctgaggtta cagagaagct agatgtggtg gaagttgaa SEQ ID NO: 6 prsAPX stromal  atggcagagc gtgtgtctct cacactcaac  maervsltln gtllsppptt (AT4G08390) ascorbate ggaaccctcc tttctcctcc tcccacaaca  ttttmssslr sttaaslllr peroxidase acaacaacaa caatgtcttc ttctctccga  ssssssrSTL tctaccaccg ccgcttctct tctcctccgc SEQ ID NO: 16 tcctcctcct cctcctccag atccactctc SEQ ID NO: 7 prTic40-E2A Translocon  atggcgaatc ttaacttagc ccttgtttct  manlnlalvs spkplllghs (AY157668  at the inner  tcccctaaac ccctgctttt aggacattcc  ssknvfsgrk sftfgtfrvs variant) envelope tcctcaaaaa acgttttctc aggaaggaag ansssshvtr aaskshqnlk membrane of tctttcactt ttgggacgtt tcgcgtttct svqgkvnahd FAS chloroplasts gctaactctt catcctctca tgtcaccagg  SEQ ID NO: 17 40 kD gctgcttcta aatctcacca aaatctaaaa  tctgtgcagg ggaaggtgaa tgcgcatgat  tttgctagc variant SEQ ID NO: 8 prCpn10-1- chaperonin  atggcttcca ctttcgtctc tctaccaaat  mastfvslpn pffafpvkat ΔC7C37S  10-1 cctttctttg cttttccggt caaagcaact  tpstanhtll gsrrgslrik (AT2G44650 variant actccttcga cggctaacca tacgcttctc  AIS variant) ggaagtcgaa gaggttccct tagaatcaaa SEQ ID NO: 18 gcgatttcc SEQ ID NO: 9 prRBCS Ribulose- atggcttcct caatgatctc ctccccagct  massmisspa vttvnragag (NM_ 1,5-bisphos- gttaccaccg tcaaccgtgc cggtgccggc  mvapftglks magfptrktn 001248385) phate atggttgctc cattcaccgg cctcaaatcc  nditsiasng grvqcMQV carboxylase atggctggct tccccacgag gaagaccaac SEQ ID NO: 20 small aatgacatta cctccattgc tagcaacggt  subunit ggaagagtac aatgcatgca ggtg SEQ ID NO: 10

Example 7 Plasmid Construction for In Vivo Expression of Fusion Proteins in Tobacco BY-2 Suspension Culture Cells and Rice Calli

Plasmids for transient in vivo expression were prepared as follows. The coding region corresponding to the transit peptide and the first three amino acids in the mature region of prRBCS, prTic40, prCpn10-2, prFibrillin, prAPS1, prAPS3, prAPR3, prsAPX, prTic40-E2A, prCpn10-1-ΔC7C37S (Table 2) were amplified by PCR using a forward primer that added a XbaI site and a reverse primer that added a BamHI site to the amplified fragment. The PCR fragment for each transit peptide was digested and cloned into the XbaI/BamHI site of the plasmid p326GFP (Lee et al., 2002), resulting in translational fusion of the transit peptide to the N terminus of GFP. The sequence was confirmed by sequencing and the plasmid was named prRBCS_(tp)-GFP, prTic40_(tp)-GFP, prCpn10-2_(tp)-GFP, prFibrillin_(tp)-GFP, prAPS1_(tp)-GFP, prAPS3_(tp)-GFP, prAPR3_(tp)-GFP, prsAPX_(tp)-GFP, prTic40-E2A_(tp)-GFP and prCpn10-1-ΔC7C37S_(tp)-GFP, respectively. The transit peptide-GFP fusion constructs were placed under the control of the cauliflower mosaic virus 35S (CaMV35S) promoter and the nopaline synthase (nos) terminator. These transit peptide-GFP fusion constructs were co-transformed with the plasmid pBI221, which contains CaMV35S driven β-glucuronidase (GUS) gene and serves as an internal control for transformation and protein expression efficiency. Protoplasts isolated from tobacco BY-2 suspension cells were transformed by polyethylene glycol mediated transformation. The amounts of GFP and GUS protein produced were determined by immunoblotting. The efficiency of each transit peptide was calculated by the amount of GFP produced normalized to the amount of GUS produced for each transformation. Rice embryo-induced calli were transformed by particle bombardment and Agrobacteria-mediated transformation. Multiple transformation experiments were performed for each construct and the average efficiency of each transit peptide was calculated and compared to the average efficiency of prRBCS transit peptide. The subcellular localization of the expressed proteins was confirmed by confocal microscopy.

Example 8 Demonstration of Transit Peptide Efficiency by In Vivo Expression of Fusion Proteins

The present invention further fused the transit peptides from the nine high-import-efficiency precursors to GFP as described above and tested the in vivo leucoplast import efficiency of these nine fusion proteins. The present invention used transient expression in tobacco BY-2 suspension culture cells and rice calli, which are widely used and represent established model systems for non-green leucoplast-containing tissues of dicotyledon and monocotyledon plants, respectively. Both systems can be easily transformed and served as a quick and reliable tool for evaluating the in vivo leucoplast import efficiency of the fusion proteins. The results (FIGS. 4 A and B) showed that, when GFP was fused to eight of nine transit peptides of the present invention, the amount of GFP imported into leucoplasts was much higher than when GFP was fused to the transit peptide of prRBCS. One of the transit peptides of the present invention had a similar efficiency to the prRBCS transit peptide.

Example 9 Plasmid Construction and Protein Expression in Transgenic Arabidopsis Plants

The DNA fragment encoding GFP from the plasmid p326GFP (Lee et al., 2002) was amplified by PCR using a forward primer that added a BamHI site and a reverse primer that added an XbaI site to the amplified fragments. The fragment was digested and cloned into the BamHI/XbaI site of pSP72. The sequences were confirmed by sequencing and the plasmid was named pSP72-GFP. The DNA fragments encoding the transit peptide and the first three amino acids of the mature region of prRBCS, prTic40, prCpn10-2, prFibrillin, prAPS3, prAPR3, prsAPX, prTic40-E2A, prCpn10-1-ΔC7C37S (Table 2) were amplified by PCR using a forward primer that added a SacI site and a reverse primer that added a BamHI site to the amplified fragment. The fragments were digested and cloned into the SacI/BamHI site of pSP72-GFP to generate the transit peptide-GFP fusion constructs. The DNA fragment encoding the transit peptide and the first three amino acids of the mature region of prAPS1 was amplified by PCR using a forward primer that added a KpnI site and a reverse primer that added a BamHI site to the amplified fragment. The fragments were digested and cloned into the KpnI/BamHI site of pSP72-GFP. All sequences were confirmed by sequencing. The DNA fragments encoding the transit peptide-GFP were excised by SacI/PstI (for prRBCS_(tp)-GFP, prTic40_(tp)-GFP, prCpn10-2_(tp)-GFP, prFibrillin_(tp)-GFP, prAPS3_(tp)-GFP, prAPR3_(tp)-GFP, prsAPX_(tp)-GFP, prTic40-E2A_(tp)-GFP and prCpn10-1-ΔC7C37S_(tp)-GFP) and KpnI/PstI (for prAPS1_(tp)-GFP) and cloned into the SacI/PstI site and KpnI/PstI site of the binary vector pCHF1 (Hajdukiewicz et al., 1994), respectively. The transit peptide-GFP fusion constructs were placed under the control of the CaMV35S promoter and the RBCS terminator. The resulting plasmids were transformed into Agrobacterium tumefaciens GV3101. Arabidopsis (Columbia ecotype) plants will be transformed by the floral spray method (Chung et al., 2000). Transgenic plants harboring the introduced transit peptide-GFP fusion transgene will be identified on MS medium containing 100 μg/mL G418. Multiple independent transgenic plants will be obtained for each transit peptide-GFP construct. The RNA and protein amount of the expressed GFP will be detected by quantitative RT-PCR and immunoblotting. The localization of GFP in root leucoplasts will be verified by confocal microscopy. The size of GFP should also be the processed mature-size GFP on immunoblots, corroborating with delivery to plastids and removal of the transit peptide. After normalization to the GFP RNA level, the efficiency of each transit peptide in delivering GFP to root leucoplasts in each transgenic plant will be calculated. The average efficiency of each transit peptide will be compared to the average efficiency of prRBCS transit peptide. The efficiency of each transit peptide in delivering GFP to leaf chloroplasts will also be compared to that of prRBCS transit peptide. It is expected that the efficiency of each of the high-efficiency transit peptides as described herein, in delivering GFP to chloroplasts, will be equal or higher than that of prRBCS transit peptide. It is further expected that the efficiency of each of the high-efficiency transit peptides as described herein, in delivering GFP to leucoplasts, will be higher than that of prRBCS transit peptide.

The present invention provides a group of nine transit peptides that have very high import efficiency into both chloroplasts and leucoplasts. They are transit peptides of the precursors of translocon at the inner envelope membrane of chloroplasts 40 kD (prTic40), chaperonin 10-2 (prCpn10-2), Fibrillin 1B (prFibrillin), ATP sulfurylase 1 (prAPS1), ATP sulfurylase 3 (prAPS3), 5′-adenylylsulfate reductase 3 (prAPR3), stromal ascorbate peroxidase (prsAPX), prTic40-E2A (a prTic40 variant) and prCpn10-1-ΔC7C37S (a chaperonin 10-1 variant). These nine transit peptides of the present invention all have similarly high chloroplast-import efficiency, and much higher leucoplast-import efficiency, than the prRBCS transit peptide. The more than 60 precursors the present invention tested and the nine precursors of the present invention have never been tested for leucoplast import before. The transit peptide of prRBCS is the most widely used transit peptide for delivering engineered proteins into plastids. Examples include the famous Golden Rice, Roundup Ready® corn and Dicamba resistant soybean. In the case of Golden rice, for example, they have used the prRBCS transit peptide to deliver the bacterial carotene desaturase into rice grain leucoplasts. The present invention has shown quantitatively here that, for delivering proteins into leucoplasts, the nine transit peptides of the present invention are much more efficient than the prRBCS transit peptide. If the one of ordinary skill in the art had used one of the transit peptides from the present invention, the production of provitamin A in the rice grains may be even higher. The next step is to test which of the nine transit peptides offer the highest protein import efficiency into plastids in stably transformed plants.

Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims. 

What is claimed is:
 1. A recombinant DNA molecule encoding a fusion protein, wherein said recombinant DNA molecule comprises a first DNA sequence encoding a transit peptide operably linked to a second DNA sequence encoding a passenger protein, wherein the transit peptide consists of the first 71 amino acids of SEQ ID NO:13.
 2. A DNA construct comprising the recombinant DNA molecule according to claim 1, wherein said recombinant DNA molecule is operably linked to a promoter.
 3. The DNA construct according to claim 2, wherein the promoter is functional in a plant cell.
 4. A plant material comprising the DNA construct according to claim
 3. 5. The plant material according to claim 4, wherein the plant material is selected from the group consisting of a plant cell, a plant tissue, a plant tissue culture, a callus culture and a transgenic plant.
 6. The plant material according to claim 4, wherein the plant material is obtained from a monocotyledon or a dicotyledon plant.
 7. A method of delivering a passenger protein into leucoplasts, comprising: (a) providing a recombinant DNA molecule encoding a fusion protein; (b) linking the recombinant DNA molecule operably with a promoter to form a DNA construct, wherein the promoter is functional in a plant cell; and (c) introducing the DNA construct into a plant material to express the fusion protein; wherein the recombinant DNA molecule comprises a first DNA sequence encoding a transit peptide operably linked to a second DNA sequence encoding a passenger protein, wherein the transit peptide consists of the first 71 amino acids of SEQ ID NO:13.
 8. The method according to claim 7, wherein the plant material is selected from the group consisting of a plant cell, a plant tissue, a plant tissue culture, a callus culture and a transgenic plant.
 9. The method according to claim 7, wherein the plant material is obtained from a monocotyledon or a dicotyledon plant. 